1. Field of the Invention
This invention relates to a method of a manufacturing a rotary anode type X-ray tube and an apparatus for manufacturing the same.
2. Description of the Related Art
As known well, a rotary anode type X-ray tube has a disc-shaped anode target supported by a rotary structure and a stationary structure between which a bearing portion is formed. As the target is rotated at a high speed by a rotating magnetic field generated from the electromagnetic coil of a stator arranged outside of an vacuum envelope, the electron beam emitted from a cathode is radiated on the anode target to irradiate X-rays. The bearing portions are constituted by rolling bearings, such as a ball bearing, or a dynamic pressure type sliding bearing which has bearing surfaces with spiral and helical grooves and uses a metal lubricant consisting of, e.g. gallium (Ga), or a gallium-indium-tin (Ga-In-Sn) alloy filled in a bearing gaps. Examples using the sliding bearings are disclosed in Japanese Laid-open Patent Disclosures No. 60-21463, No. 60-97536, No. 60-117531, No. 62-287555 and No. 2-227948.
In the rotary anode type X-ray tube disclosed in the Disclosures, the bearing surfaces of the dynamic sliding bearing which has spiral and helical grooves are faced to have the bearing gap of 20 micrometers or so, and a liquid metal lubricant is filled in the bearing gap and the spiral and helical grooves. If air or gas is left in the bearing portions or is discharged from lubricant or bearing parts such as the rotary structure and stationary structure, the bearing surfaces become oxidized and are not wetted well by the lubricant. In some cases, bubbles prevent some portions of the bearing surfaces from being coated with liquid metal lubricant. When the assembled X-ray tube is operated, the temperature of the bearing portions rises to about 200.degree. C. Thus, gas is sometimes discharged from the bearing portions and the lubricant during the operation of the X-ray tube so that the gas may cause the lubricant to blow up with gas or bubbles from the bearing portions. When this phenomenon occurs, the bearing can not perform a stable dynamic pressure type sliding bearing operation for a long time, and the withstand voltage property of the X-ray tube is significantly deteriorated due to the the liquid metal lubricant scattered into the X-ray tube envelope.